1. Field of the Invention
The present invention relates to the technologies used especially in homeland security or nuclear safeguard applications, e.g. where security relevant nuclear, radiating materials passing the border must be detected and/or characterized. It refers to a method for obtaining information signatures from nuclear material or about the presence, the nature and/or the shielding of a nuclear material.
It further refers to a measurement setup for performing such a method.
2. Description of Related Art
The capability of detecting special nuclear materials (SNM) is of importance in the context of preventing and deterring misuse of nuclear material. In particular at ports of entry, there is an interest to prevent such material from being illicitly brought into a country. For this purpose radiation detectors of different form factors can be used, including large, vehicle-sized radiation portal monitors (RPM, see FIG. 3). The following description uses RPMs as an example, analogous principles apply equally for other radiation detectors such as for example handheld or portable devices.
RPMs are effective at detecting radiological materials suitable for misuse in a radiological dispersal device (RDD), for example a dirty bomb. This is not surprising, since a substantial amount of radioactivity is required for such a device to have its desired effect, the high level of radioactivity typically giving a clear signature of gamma radiation.
The detection of SNM is complicated by the fact that SNM is not highly radioactive. Depending on its nature, SNM can in some (but not all cases) emit substantial amounts of neutron and/or gamma radiation. Even so, the intensity of radiation emitted may be low compared to other backgrounds, including natural backgrounds or benign sources such as naturally occurring radioactive materials (NORM). In cases where SNM is additionally shielded by neutron or gamma shielding, the detection problem becomes very difficult to solve.
Typically, RPMs deployed with the intention of interdicting SNM aim at detecting both neutron and gamma radiation signatures from fission: Neutron signatures are detected using thermal neutron detectors surrounded by moderating material; gamma signatures are detected by scintillation detectors such as polyvinyltholuene (PVT). Faced with the challenge of detecting weak radiation signatures characteristic of fission in the presence of strong backgrounds, RPMs have so far not been able to deliver high SNM detection capabilities without incurring unacceptably high ‘nuisance alarm’ rates caused by legitimate NORM sources in commerce.
In an attempt to improve detection probability and reduce nuisance alarm rates, PVT scintillators were replaced by higher resolution gamma detectors such as sodium iodide (NaI) scintillation crystals or high purity germanium detectors. In field trials, RPMs based on these technologies were not capable of demonstrating a substantial improvement in achieving the goals set out.